Field of the Invention
The present invention relates to the technical field of stress measurements of a wellbore. More specifically it relates to the field of determining wellbore formation instability by measuring stress outside a wellbore conduit and analysing resulting stress parameters.
Description of Prior Art
Instabilities in wellbores can have serious consequences, such as fracturing or collapse of the wellbore. Instabilities can be caused by changes in the surrounding formation due to e.g. erosion, and washout which will again lead to in-situ stresses. In some wellbores, and especially wellbores with casing strings cemented in place, the stresses may build up over time, apparently without influencing drilling operations. However, due to e.g. washout and erosion outside the casing wall, stresses may build up, leading to a potentially dangerous situation. Collapse of a wellbore can have both large economic and environmental consequences. It is therefore important to monitor and analyze changes in stresses outside the casing to be able to prevent such situations from happening as a result of instabilities.
The formation surrounding an oil well may be composed of different materials, typically rock and sediments, as well as fluids.
When a load is applied to the formation, it is carried by the solid particles as well as the fluid in the pores. The flow rate of the fluid depends on the permeability of the formation, whereas the strength and compressibility of the soil depend on the stresses between the solid particles of the formation.
The total vertical stress acting at a point in the formation is due to forces from any material or water above the point, i.e. particles, water, and other loads.
Vertical stress will be related to horizontal stress through complex relationships, and changes in the vertical stress will influence the horizontal stress and vice-versa.
Local formation changes and instabilities can be caused by changes in total stress, e.g. changes in load due to depletion of nearby reservoirs etc.
However, these formation changes and instabilities can also be caused by changes in pore pressures.
As an example, consider sand that initially is damp. It will remain intact because the pore pressure is initially negative, but as it dries, this pore pressure suction is lost and it collapses.
It is therefore not sufficient only to understand how the total stress is acting on the formation. More importantly, the combinatory effect that total stress and pore pressure should be used when analysing e.g. stability of the formation. This combined parameter is termed effective stress and it is given as the difference between the total stress and the pore pressure.
Effective stress controls shear strength, compression, distortion changes in strength, changes in volume, changes in shape etc. of the formation.
Effective stress represents the distribution of load carried by the soil over the area considered.
Total and effective stresses should be handled separately. Movements and instabilities can be caused by changes in total stress, such as loading by foundations and unloading due to slides. They can also be caused by changes in pore pressure. Sudden changes in wellbore stress may be caused by sudden fluid movements on the outside of the wellbore, thermal stress with time that is induced by either production of injection, sudden change in overburden pressures, compactions of formation related to depletion of underlying formations, etc.
The critical shear strength of the formation is a function of the effective normal stress and a change in the effective stress will lead to a change in strength.
In US patent application 2012173216 A1 logging data from geophysical surveys are used to determine stresses in the wellbore for the purpose of discovering subterranean assets.
U.S. Pat. No. 5,285,692 describes calculation of mean effective stress around the wellbore using geostatic overburden in situ stress, the field pore pressure and the total stress around the wellbore based on shale cuttings.
Various methods exist for collecting data from an in-situ location. GB 2466862 A describes in situ measurements of wellbore and formation parameters, and communication of the signals over a wireless link.
However, the problem remains of how to effectively provide continuous determination of wellbore stability in a specific area of a wellbore based on analysis of real measurements.